Timeline
The following is a timeline of significant events and introductions of games in the Sonic the Hedgehog series. Note that some of the earlier games may not be placed at any specific time in the timeline, leaving it up to viewers to decide when some of them take place. *Sonic the Hedgehog - Sonic the Hedgehog and Dr. Robotnik are introduced *Sonic the Hedgehog 2 - Tails is introduced, as are the Chaos Emeralds and the Death Egg *Sonic the Hedgehog 3 - Knuckles and Angel Island are introduced *Sonic & Knuckles - Knuckles becomes Sonic's ally for the first time, and Sonic destroys the Death Egg *Sonic the Hedgehog 4 Episode I - Metal Sonic is created and has his first chronological appearance *Sonic CD - Amy Rose is introduced, and Metal Sonic is officially introduced as well (Note: Some believe this game is set before Sonic the Hedgehog 2) *Sonic the Hedgehog 4 Episode II - Metal Sonic is revived from his previous defeat *SegaSonic the Hedgehog - Mighty the Armadillo is introduced, and Ray the Flying Squirrel has his only appearance *Tails Adventure - Tails defeats the one-shot villain Grand Battle Kukku 15th (Note: It is unclear where this game is placed in the timeline) *Knuckles' Chaotix - Final appearance of Mighty, and first appearance of Espio the Chameleon, Charmy Bee, and Vector the Crocodile (Where this is set in the timeline is also unclear) *Sonic Chaos - No significant events *Sonic Triple Trouble - First appearance of Nack the Weasel *Sonic Labyrinth - No significant events *Sonic the Fighters - Final appearance of Nack/Fang, and only appearance of Bark the Polar Bear and Bean the Dynamite *Sonic 3D Blast - No significant events *Sonic R - Last game in the classic era *Sonic Adventure - Sonic calls Robotnik "Eggman", Big the Cat is introduced, and E-102 Gamma and Chaos have their only true appearances (though Chaos's battle is revisited and he is referenced) *Sonic Adventure 2 - Shadow the Hedgehog, Rouge the Bat, and the President are introduced *Sonic Advance - No significant events *Sonic Advance 2 - Cream the Rabbit and her mother Vanilla the Rabbit are introduced *Sonic Advance 3 - Only appearance of one-shot villain Gemerl *Sonic Heroes - E-123 Omega is introduced; Final mainstream appearance of Big; Metal Sonic is main villain for the first time *Shadow the Hedgehog - Shadow's full backstory is revealed; Only appearance of one-shot villain, Black Doom *Sonic Riders - First appearance of Jet the Hawk, Wave the Swallow, and Storm the Albatross *Sonic Riders: Zero Gravity - Only appearance of one-shot villain SCR-HD; The Babylonians' backstory is explained *Sonic Free Riders - Metal Sonic returns for a rematch with Sonic *Sonic Rush - First appearance of Blaze the Cat and Eggman Nega *Sonic Rush Adventure - Only appearance of Marine the Raccoon and one-shot villains Captain Whisker and Johnny *Sonic the Hedgehog (2006) - Silver the Hedgehog is introduced; only appearance of Princess Elise and one-shot villains Iblis and Mephiles the Dark; The events of this game are erased from the timeline *Sonic Rivals - Sonic and Silver canonically meet for the first time *Sonic Rivals 2 - Final appearance of Eggman Nega, and only time Metal Sonic is a protagonist *Sonic Unleashed - Orbot is introduced; Only appearance of Chip, Professor Pickle, and the one-shot villain Dark Gaia *Sonic Colors - Cubot is introduced; Only appearance of Yacker; Eggman is trapped in space to set up the events of Generations. *Sonic Generations - Metal Sonic is destroyed *Sonic Lost World - Only appearance of the one-shot villains, the Deadly Six.